feat: add 54 financial and engineering formula functions

[Nebu1eto]

Summary

• Add 21 financial functions: FV, PV, NPV, IRR, PMT, IPMT, PPMT, RATE, NPER, DB, DDB, SLN, SYD, EFFECT, NOMINAL, DOLLARDE, DOLLARFR, CUMIPMT, CUMPRINC, XNPV, XIRR
• Add 33 engineering functions: BIN2DEC, BIN2HEX, BIN2OCT, DEC2BIN, DEC2HEX, DEC2OCT, HEX2BIN, HEX2DEC, HEX2OCT, OCT2BIN, OCT2DEC, OCT2HEX, DELTA, GESTEP, ERF, ERFC, COMPLEX, IMREAL, IMAGINARY, IMABS, IMARGUMENT, IMCONJUGATE, IMSUM, IMSUB, IMPRODUCT, IMDIV, IMPOWER, IMSQRT, CONVERT, BESSELI, BESSELJ, BESSELK, BESSELY
• 80 new tests (30 financial + 50 engineering), total supported formula functions: 164
• Updated EN and KO documentation

Test plan

• cargo build --workspace passes
• cargo test --workspace passes (1508 tests total, 80 new)
• cargo clippy --workspace passes (no warnings)
• cargo fmt --check passes
• pnpm build && pnpm vitest run passes (254 Node.js tests)
• EN docs updated (api-reference/advanced.md, guide/advanced.md)
• KO docs updated (api-reference/advanced.md, guide/advanced.md)

🤖 Generated with Claude Code

[Nebu1eto]

Completed a formula-function pass focused on integration points and tests. No blocking issue found in this review round.

[Nebu1eto]

Detailed review notes:

The function expansion is substantial and test coverage is broad. The implementation style aligns with existing formula engine patterns.

What looks good:

• Clear function grouping and consistent evaluator integration.
• Strong volume of examples and expectation tests across new functions.

Suggested hardening:

• Add explicit edge tests for extreme numeric values (very large/small magnitudes) where floating-point instability can surface.
• Add dedicated behavior tests for error propagation order (for example invalid arg type vs range error precedence).
• Consider adding compatibility snapshots for a small set of formulas against Excel outputs to reduce semantic drift risk over time.